Today, I have guest author Allison Bailes from Energy Vanguard sitting in for me. I specifically asked him to cover this topic since the question comes up so often.

Since I’ve acknowledged myself as the best, baddest, building science blogger of the internets, I get asked a lot of questions about windows, air conditioning, and other energy efficiency topics: What size HVAC system should I install? Is it true you can seal a house up too tight? Can I use a CFL bulb in my Easy-Bake Oven?

I also get asked about insulation, and that’s today’s topic. I’m going to stick to the attic here, but much of what I say could apply to walls or floors, too.

The first thing to know is that you really have only three choices here. Well, OK, you have more than three, but I’m mostly going to talk about those three because in terms of what you’ll be able to find someone to install, these are the ones you’re mostly limited to.

Before you ever get insulation anywhere near the attic, though, make sure that you get the air leakage sites sealed up. If you put an air permeable insulation material over a hole in your ceiling, you may have comfort, indoor air quality, durability, and efficiency problems.

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Batts

These are large pieces of insulation that hold together because they’re made of long, interweaving fibers with adhesive binders. The two kinds of batts you’re most likely to encounter are fiberglass and cotton. In terms of their insulating quality, they’re pretty much equivalent. Cotton batts, though, are ‘cool’ because they’re made of recycled blue jeans.

The problem with batts, however, is that they don’t work well because they don’t fill the space well. For the best performance, an insulation material needs to fill the whole space, with no gaps, voids, compression, or incompletely filled areas. Batts are about the worst you can do here.

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See that photo above? Notice that you don’t see insulation filling all the spaces between the ceiling joists. In this case, it’s because they weren’t cut to fill the cavity completely. Another reason that batts don’t do so well is that the house is full of other stuff where we want the insulation to go: wires, electrical junction boxes, framing, bathroom exhaust fans, can lights… Batts don’t do well when they have to compete against all that.

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Blown

A better choice is insulation that comes in smaller chunks. The installer, taking his best firefighter pose, holds a large hose and blows the chunks into the attic. A large machine outside churns the chunks and uses air to blow them up through the hose.

The two main choices here are fiberglass and cellulose, and each has its advantages and disadvantages. They both insulate about the same, though, with R-vales in the 3 to 4 per inch range. Cellulose comes from recycled newspapers. Fiberglass comes from what I’ve heard one major fiberglass insulation manufacturer call a ‘rapidly renewable’ resource – sand. Hmmmm. I don’t know about that, but it’s a common insulation material that works much better in the blown form than in batts.

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The photo above shows an attic insulated with blown cellulose. Notice how you don’t see any of the ceiling framing down at the ceiling level. You also don’t see any gaps that allow you to see all the way down to the ceiling drywall. That’s because blown insulation is great at filling the gaps and giving you a good, complete layer of insulation.

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Sprayed

The third major type of insulation is spray foam. Just as there are two types of blown insulation (fiberglass and cellulose) and two types of people (those who divide everything into two groups and those who don’t), there are two types of spray foam – open cell and closed cell. Each has its pros and cons, as well as its own set of adherents who will tell you never to use the other type. That’s an article for another day, however.

The main advantage of spray foam is that it allows you to move the building envelope - the boundary between conditioned and unconditioned space – from the attic floor to the roof line. If you’ve got your HVAC system and ducts in the stupidest place they could possibly be  (the attic), then moving the envelope to the roofline can be a good thing. In a new home, spraying foam in the roofline can bring the ducts inside the envelope without having to redesign the system and house.

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If you don’t have HVAC and ducts in the attic, spray foam on the roofline isn’t really necessary. I’d blow insulation on the attic floor (after air-sealing, of course). The big disadvantage with spray foam is cost. It’s generally 3 to 4 times what you’ll pay for blown cellulose or fiberglass.

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Alternative Materials

There are other materials and systems that you can use to insulate your attic. One that I like a lot (since I built a house out of them  is the structural insulated panel. It’s a sandwich of rigid foam insulation and plywood or OSB (oriented strand board, the flaky plywood).

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One that I don’t like so much, and which you can see below, is the beer can.

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